collections.abc — Abstract Base Classes for Containers

New in version 3.3: Formerly, this module was part of the collections module.

Source code: Lib/_collections_abc.py

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This module provides abstract base classes that can be used to test whether a class provides a particular interface; for example, whether it is hashable or whether it is a mapping.

An issubclass() or isinstance() test for an interface works in one of three ways.

1) A newly written class can inherit directly from one of the abstract base classes. The class must supply the required abstract methods. The remaining mixin methods come from inheritance and can be overridden if desired. Other methods may be added as needed:

class C(Sequence):                      # Direct inheritance
    def __init__(self): ...             # Extra method not required by the ABC
    def __getitem__(self, index):  ...  # Required abstract method
    def __len__(self):  ...             # Required abstract method
    def count(self, value): ...         # Optionally override a mixin method

>>> issubclass(C, Sequence)
True
>>> isinstance(C(), Sequence)
True

2) Existing classes and built-in classes can be registered as “virtual subclasses” of the ABCs. Those classes should define the full API including all of the abstract methods and all of the mixin methods. This lets users rely on issubclass() or isinstance() tests to determine whether the full interface is supported. The exception to this rule is for methods that are automatically inferred from the rest of the API:

class D:                                 # No inheritance
    def __init__(self): ...              # Extra method not required by the ABC
    def __getitem__(self, index):  ...   # Abstract method
    def __len__(self):  ...              # Abstract method
    def count(self, value): ...          # Mixin method
    def index(self, value): ...          # Mixin method

Sequence.register(D)                     # Register instead of inherit

>>> issubclass(D, Sequence)
True
>>> isinstance(D(), Sequence)
True

In this example, class D does not need to define __contains__, __iter__, and __reversed__ because the in-operator, the iteration logic, and the reversed() function automatically fall back to using __getitem__ and __len__.

3) Some simple interfaces are directly recognizable by the presence of the required methods (unless those methods have been set to None):

class E:
    def __iter__(self): ...
    def __next__(next): ...

>>> issubclass(E, Iterable)
True
>>> isinstance(E(), Iterable)
True

Complex interfaces do not support this last technique because an interface is more than just the presence of method names. Interfaces specify semantics and relationships between methods that cannot be inferred solely from the presence of specific method names. For example, knowing that a class supplies __getitem__, __len__, and __iter__ is insufficient for distinguishing a Sequence from a Mapping.

New in version 3.9: These abstract classes now support []. See Generic Alias Type and PEP 585.

Collections Abstract Base Classes

The collections module offers the following ABCs:

ABC	Inherits from	Abstract Methods	Mixin Methods
Container [1]		__contains__
Hashable [1]		__hash__
Iterable [1] [2]		__iter__
Iterator [1]	Iterable	__next__	__iter__
Reversible [1]	Iterable	__reversed__
Generator [1]	Iterator	send, throw	close, __iter__, __next__
Sized [1]		__len__
Callable [1]		__call__
Collection [1]	Sized, Iterable, Container	__contains__, __iter__, __len__
Sequence	Reversible, Collection	__getitem__, __len__	__contains__, __iter__, __reversed__, index, and count
MutableSequence	Sequence	__getitem__, __setitem__, __delitem__, __len__, insert	Inherited Sequence methods and append, reverse, extend, pop, remove, and __iadd__
ByteString	Sequence	__getitem__, __len__	Inherited Sequence methods
Set	Collection	__contains__, __iter__, __len__	__le__, __lt__, __eq__, __ne__, __gt__, __ge__, __and__, __or__, __sub__, __xor__, and isdisjoint
MutableSet	Set	__contains__, __iter__, __len__, add, discard	Inherited Set methods and clear, pop, remove, __ior__, __iand__, __ixor__, and __isub__
Mapping	Collection	__getitem__, __iter__, __len__	__contains__, keys, items, values, get, __eq__, and __ne__
MutableMapping	Mapping	__getitem__, __setitem__, __delitem__, __iter__, __len__	Inherited Mapping methods and pop, popitem, clear, update, and setdefault
MappingView	Sized		__len__
ItemsView	MappingView, Set		__contains__, __iter__
KeysView	MappingView, Set		__contains__, __iter__
ValuesView	MappingView, Collection		__contains__, __iter__
Awaitable [1]		__await__
Coroutine [1]	Awaitable	send, throw	close
AsyncIterable [1]		__aiter__
AsyncIterator [1]	AsyncIterable	__anext__	__aiter__
AsyncGenerator [1]	AsyncIterator	asend, athrow	aclose, __aiter__, __anext__

Footnotes

[1] (1,2,3,4,5,6,7,8,9,10,11,12,13,14)

These ABCs override object.__subclasshook__() to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[2]

Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an __iter__() method, but it does not detect classes that iterate with the __getitem__() method. The only reliable way to determine whether an object is iterable is to call iter(obj).

Collections Abstract Base Classes – Detailed Descriptions

class collections.abc.Container

ABC for classes that provide the __contains__() method.

class collections.abc.Hashable

ABC for classes that provide the __hash__() method.

class collections.abc.Sized

ABC for classes that provide the __len__() method.

class collections.abc.Callable

ABC for classes that provide the __call__() method.

class collections.abc.Iterable

ABC for classes that provide the __iter__() method.

Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an __iter__() method, but it does not detect classes that iterate with the __getitem__() method. The only reliable way to determine whether an object is iterable is to call iter(obj).

class collections.abc.Collection

ABC for sized iterable container classes.

New in version 3.6.

class collections.abc.Iterator

ABC for classes that provide the __iter__() and __next__() methods. See also the definition of iterator.

class collections.abc.Reversible

ABC for iterable classes that also provide the __reversed__() method.

New in version 3.6.

class collections.abc.Generator

ABC for generator classes that implement the protocol defined in PEP 342 that extends iterators with the send(), throw() and close() methods. See also the definition of generator.

New in version 3.5.

class collections.abc.Sequence

class collections.abc.MutableSequence

class collections.abc.ByteString

ABCs for read-only and mutable sequences.

Implementation note: Some of the mixin methods, such as __iter__(), __reversed__() and index(), make repeated calls to the underlying __getitem__() method. Consequently, if __getitem__() is implemented with constant access speed, the mixin methods will have linear performance; however, if the underlying method is linear (as it would be with a linked list), the mixins will have quadratic performance and will likely need to be overridden.

Changed in version 3.5: The index() method added support for stop and start arguments.

class collections.abc.Set

class collections.abc.MutableSet

ABCs for read-only and mutable sets.

class collections.abc.Mapping

class collections.abc.MutableMapping

ABCs for read-only and mutable mappings.

class collections.abc.MappingView

class collections.abc.ItemsView

class collections.abc.KeysView

class collections.abc.ValuesView

ABCs for mapping, items, keys, and values views.

class collections.abc.Awaitable

ABC for awaitable objects, which can be used in await expressions. Custom implementations must provide the __await__() method.

Coroutine objects and instances of the Coroutine ABC are all instances of this ABC.

Note

In CPython, generator-based coroutines (generators decorated with types.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Awaitable) for them will return False. Use inspect.isawaitable() to detect them.

New in version 3.5.

class collections.abc.Coroutine

ABC for coroutine compatible classes. These implement the following methods, defined in Coroutine Objects: send(), throw(), and close(). Custom implementations must also implement __await__(). All Coroutine instances are also instances of Awaitable. See also the definition of coroutine.

Note

In CPython, generator-based coroutines (generators decorated with types.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Coroutine) for them will return False. Use inspect.isawaitable() to detect them.

New in version 3.5.

class collections.abc.AsyncIterable

ABC for classes that provide __aiter__ method. See also the definition of asynchronous iterable.

New in version 3.5.

class collections.abc.AsyncIterator

ABC for classes that provide __aiter__ and __anext__ methods. See also the definition of asynchronous iterator.

New in version 3.5.

class collections.abc.AsyncGenerator

ABC for asynchronous generator classes that implement the protocol defined in PEP 525 and PEP 492.

New in version 3.6.

Examples and Recipes

ABCs allow us to ask classes or instances if they provide particular functionality, for example:

size = None
if isinstance(myvar, collections.abc.Sized):
    size = len(myvar)

Several of the ABCs are also useful as mixins that make it easier to develop classes supporting container APIs. For example, to write a class supporting the full Set API, it is only necessary to supply the three underlying abstract methods: __contains__(), __iter__(), and __len__(). The ABC supplies the remaining methods such as __and__() and isdisjoint():

class ListBasedSet(collections.abc.Set):
    ''' Alternate set implementation favoring space over speed
        and not requiring the set elements to be hashable. '''
    def __init__(self, iterable):
        self.elements = lst = []
        for value in iterable:
            if value not in lst:
                lst.append(value)

    def __iter__(self):
        return iter(self.elements)

    def __contains__(self, value):
        return value in self.elements

    def __len__(self):
        return len(self.elements)

s1 = ListBasedSet('abcdef')
s2 = ListBasedSet('defghi')
overlap = s1 & s2            # The __and__() method is supported automatically

Notes on using Set and MutableSet as a mixin:

1. Since some set operations create new sets, the default mixin methods need a way to create new instances from an iterable. The class constructor is assumed to have a signature in the form ClassName(iterable). That assumption is factored-out to an internal classmethod called _from_iterable() which calls cls(iterable) to produce a new set. If the Set mixin is being used in a class with a different constructor signature, you will need to override _from_iterable() with a classmethod or regular method that can construct new instances from an iterable argument.

2. To override the comparisons (presumably for speed, as the semantics are fixed), redefine __le__() and __ge__(), then the other operations will automatically follow suit.

3. The Set mixin provides a _hash() method to compute a hash value for the set; however, __hash__() is not defined because not all sets are hashable or immutable. To add set hashability using mixins, inherit from both Set() and Hashable(), then define __hash__ = Set._hash.

See also

• OrderedSet recipe for an example built on MutableSet.

• For more about ABCs, see the abc module and PEP 3119.